Global financial markets dropped sharply Monday in reaction to an unprecedented downgrade of the U.S. government's credit rating. U.S. stock markets dropped more than 3 percent in the first hours of trading. Major indexes across Asia closed between 2 percent and nearly 4 percent lower, and European stocks were down more than 3 percent in mid-day trading.Monday trading sessions were the first chance for many investors to react to the change in the Standard & Poor's rating, which was announced on Friday evening after world markets had closed.The rating agency held a conference call today to defend its decision. S&P Managing Director John Chambers, head of the committee that made the ratings decision, said increasing debt levels in the United States and the inability of Congress and the president to reach political consensus for significant deficit reductions were no longer comparable with the most highly rated governments. U.S. leaders have criticized the S&P downgrade, which signals that the agency believes U.S. government bonds are now a riskier investment. U.S. Treasury Secretary Timothy Geithner said the S&P used terrible judgment when it downgraded the U.S. credit rating last week from the top triple-A grade.